Abstract
KEYWORDS – third body, friction mechanisms, brake pad materials, tribological contact, wear
ABSTRACT
Research and /or Engineering Questions/Objective:
During braking, third-body flows and layers govern friction mechanisms, which are fully responsible of the friction coefficient and wear. In the context of development of brake friction pairs, the involved tribological circuit has to be well understood and mastered. The difficulty to study the third body is that there is no simple relationship existing between composition of the friction layer and bulk material formulation. This review gives some leads in order to better understand and predict the friction behaviour of brake pad materials.
Methodology and Results:
This review is divided in two main parts: The first one is about the formation mechanisms. The first formation factor during the tribological test is wear, which occurs at different length and time scales by different processes. If we consider the work of Jacobson et al., friction layers are discontinuous comprising of primary and secondary contact patches, called ‘plateaus’. They can be spotted with the naked eye as shiny spots against a dark background. Further, stressing of the system may lead to continuous destruction and restoration of these contact patches as well as to an alteration of the structure and properties of the third body with changing environmental conditions. The major wear mechanism is delamination of filler particles from the organic binder, supported by local degradation of the phenolic resin during asperity heating. The main chemical–physical reactions occurring during braking are due to the combination of the friction induced heat and mechanical actions such as attrition and wear. The second part is about morphology and composition of the third body/transfer layer. Morphology and composition obviously depend on the brake pads and rotor disc ingredients: The main constituent is always iron oxide, but also the nanoconstituents carbon and copper followed by Si and Ca are easily founded. Metal suphides, as showed by Osterle et al., play a fundamental role, creating thin film with mixed ingredients (Fe, Cu, ...) on the top of the phenolic resin binder. Within the deformed layer, small grains, mostly elongated in sliding direction, are visible. Severe plastic deformation, which causes such changes of the microstructure, may take place during braking.
Conclusion: Despite of the difficulties to describe and understand microscopic contact situations in respect to macroscopic properties for such a complicated component as a brake system, some general conclusions can be drawn from our researches. The third body exists at the surfaces of both counterparts and it comprises a nanocrystalline microstructure, which implies that investigations on the nanometer scale are essential for understanding the frictional behaviour of such contacts. 2 Micro contact areas or contact patches will not be stable during tribological stressing but a dynamic equilibrium between initiation, growth and degradation of contact patches will take place, as proved by Ostermeyer. As others correlated phenomena, mechanical mixing oxidation plays a major role during debris production and friction layer formation. The composition of friction films on both pad and rotor is mainly determined by solid lubricants that are part of the pad formulation.